Optical choppers are mechanical devices that physically block a
light beam of some type.

Rotating optical choppers are perhaps the most common form and
are the type produced by Scitec Instruments Ltd. A metal disc with slots etched
into it is mounted on a DC motor and rotated. The disc is placed in the path of
the light beam which will then cause the beam to be periodically interrupted by
the blocking part of the disc.

Mechanical optical choppers are useful where it is not possible
to control the light source directly or at the speeds required. For example a
standard filament light bulb can be pulsed to a few 100 Hz though the depth of
modulation is limited. If it is required to switch the light on an off
completely at 20 kHz, then the use of a mechanical optical chopper is
required.

The motors used are not designed to operate in a vacuum and it
is therefore not recommended. However, we do know of some customers who have
successfully operated the systems in a vacuum. The main issues are the loss of
lubrication fluid causing the motor lifetime to be significantly reduced and
outgassing of the motor materials.

One option for customers is to purchase the required discs from
Scitec and then to mount them on a stepper motor / dc motor designed for vacuum
use.

If the resultant figure is greater than 1193 km/hr (or 741
miles/hr) then you will need to break the sound barrier to reach the required
chopping rate. We can almost reach half this figure with our high speed chopper in the right conditions but it makes rather
a fuss about it.

Please note that the formula does not include the diameter of
the disc. This is due to the limiting factor for the speed of optical choppers
being wind resistance. To a first approximation, the speed at the edge of the
disc is directly related to the wind resistance and is independant of the
diameter of the disc. As an example our 200mm diameter
system will only spin at approximately half the speed of our 102mm diameter system with the same power motor. Hence the
speed at the edge of the disc is the same.

If you have a large beam diameter but wish to chop at a fast
rate then there are a couple of tricks that you can use. Unfortunately, both
methods mean that you will loose at least 50% of your signal strength and you
won't get exactly 100% blocking in the dark state.

Method 1: Place a stationary disk with the same number of slots
in front (or behind) the rotating disc. As the discs go in and out of phase the
beam is either blocked or 50% is allowed through. More info here.

Method 2: Place two chopper systems with the same blades in
front of each other and rotating in opposite directions. The chopping rate is
the sum of the individual chopping rates and the maximum beam size is the slot
length rather than the slot width. Using this technique, a 10 mm beam can be
chopped at 240 kHz using our high speed chopper
system.

Our current record is 120 kHz with our high
speed system. This is about the limit with 0.25 mm thick base material
using normal photo etching processes. However, we have access to electro formed
components which allow the number of slots to be increased considerably. Using
this technology 1 to 2 MHz should be possible though the slot width will be
rather small. If you have a pot of money and would like us to make a prototype
then please get in touch.

If your optical beam has a gausian profile then it is possible
to get a fairly close approximation to a sine wave using our standard chopping
discs. Please select a disc that has a slot width that is the same or wider
than your beam. Rotate the chopper relative to the beam until both sides of the
beam just touch the chopping disc.

A square wave output can be achieved by using a disc with slot
widths that are many times wider than the beam diameter though this does
restrict the chopping rate.

Triangular output waveforms are produced by the 300HF high frequency accessory.

Other waveforms can be achieved by either mounting multiple
blades on the chopping head at one time or by producing a custom disc.
Contact us with your
requirements and we will feed them into a simulation we have to find the slot
shape required.